Phagocytosis and Encapsulation: Cellular Immune Responses in Arthropoda

The least understood aspects of the cellular immune reactions of arthropods are the earliest events: the initial recognition of foreignness, and the resulting changes in hemocyte behavior and morphology. There are indications that the recognition of a surface as foreign is based primarily upon its electrostatic charge, but more specific criteria may be utilized in some situations. Phagocytes are capable of recognizing foreignness without the intervention of soluble opsonins but, in some arthropods, there is in vitro evidence that opsonins can increase the efficiency of phagocytosis. It has been hypothesized, on the basis of ultrastructural evidence that encapsulation, a multicellular immune response, is induced when labile hemocytes rupture upon encountering a foreign object. The released products may promote the formation of a sheath of ameboid hemocytes around the object. This hypothesis is now supported by the results of experiments performed on an in vitro encapsulation system. This system may prove useful in the purification of encapsulation—promoting factors; in determining the mechanism of their release from hemocytes; and in investigating the possibility that the various cellular immune reactions of arthropods have a common underlying mechanism.